The present invention relates to a method for manufacturing electrodes for a battery, comprising a process of filling an active material into a three-dimensional porous metal substrate sheet and a process of-cutting the sheet to a certain size.
In recent years, the variety of uses for batteries has rapidly increased since they began to be used as the power sources for portable appliances such as cellular phones and personal computers as well as for electric vehicles including electric cars. With this diversification, various types of batteries are being developed and improved from the viewpoint of enhancing battery performance such as energy density and reducing the cost.
As an important technique to obtain a battery with high-energy density, it has been studied to increase the electric capacity per volume of an electrode by filling an active material into the electrode substrate at a high density. For example, as the substrates of the positive electrodes in alkaline storage batteries including nickel-cadmium storage batteries, nickel-metal hydride storage batteries, and nickel-zinc storage batteries, a sintered-type porous metal sheet and three-dimensional porous metal substrate sheets such as a foamed metal sheet (sponge metal sheet), and a porous sheet of metallic fiber are used. By using these substrates, positive electrodes having a high capacity density are being developed
The sintered-type porous metal sheet is obtained by applying a slurry containing nickel powder or some similar material onto a core sheet like a punched metal, and sintering it. By immersing the sintered sheet as the positive electrode in a nickel nitrate solution, and electrolyzing it, nickel hydroxide, which is an active material, can be precipitated inside the pores of the sintered sheet. This method is called an electrodeposition process. In order to fill the active material into the sintered sheet at higher density, a chemical impregnation process is employed for filling more nickel hydroxide into the sintered sheet. In the chemical impregnation process, an impregnation with a nickel nitrate solution and an immersion with an alkaline solution are conducted. A positive electrode obtained through these treatments excels in large current charge-discharge characteristics. However, the frame consisting of a sintered sheet including core sheet makes up about 25% of the entire volume of the sintered sheet, thereby setting limits to high-density filling of an active material.
On the other hand, a nickel positive electrode with a three-dimensional porous metal substrate sheet can have an active material thereinto at an extremely high density because of the high porosity of the substrate sheet. For example, a foamed nickel sheet has a porosity as high as 96%. This foamed nickel sheet can be obtained as follows. First, a conductive composite is applied on a foamed resin (sponge resin) sheet like polyurethane sheet, and then nickel is plated on the resin sheet. After this, the internal resin is removed by burning the sheet in a reducing atmosphere so as to provide a foamed nickel sheet.
The pores of the three-dimensional porous metal substrate sheet have a diameter of about 200 xcexcm or above, which is about 10 times as large as the pores of a sintered-type porous metal substrate sheet. Such large pores can be directly filled with a paste, which is a mixture of an active material like nickel hydroxide powder, an additive like metallic cobalt, and water, by rubbing or pressing the paste thereinto.
Thus, employing a three-dimensional porous metal substrate sheet achieves an electrode having a high capacity, and greatly simplifies a process of filling an active material thereinto. The substrate sheet is also used for hydrogen storage alloy negative electrodes in nickel-metal hydride storage batteries.
On the other hand, the substrate sheet has a drawback that the active material filled thereinto tends to come off and adhere to the surface of the sheet.
Moreover, the frame of the three-dimensional porous metal substrate sheet is composed of acicular or fibriform metal complicatedly entangled, so that cutting the substrate sheet filled with the active material is likely to leave a number of sharp-pointed burrs.
When a battery is assembled with such an electrode, burrs and adherents penetrate the separator, causing the battery to short-circuit. This problem is the main cause of a decrease in the production yield of the electrode, and, deterioration in the preservation performance and charge-discharge cycle life of the battery.
A method for solving this problem has been suggested in Japanese Laid-Open Patent Publication No. 8-45500. In the Publication, after cutting the substrate sheet filled with an active material to a certain size or after cutting the substrate sheet to a certain size and filling an active material thereinto, the periphery of the substrate is pressed or removed. The pressing of the periphery of the substrate after cutting is done for the purposes of holding down the burrs occurred during the cutting and of making the periphery of the electrode thinner. The removing of the periphery of the substrate after cutting is done for the purpose of deburring. These processes, however, cannot prevent burrs once held down from rising again, or cuttings and fallen active material from adhering to the surface of the electrode. Thus, the battery still has the risk of an internal short circuit.
On the other hand, a method for covering the periphery and cross section of an electrode with a hot melt resin has been suggested (Japanese Laid-Open Patent Publication No. 5-190200). In this method, however, the electrode sheet is cut and then its cross section is covered with the resin, which is not related to suppressing the occurrence of sharp-pointed burrs or cuttings. These burrs and cuttings penetrate the resin layer, making it hard to prevent an internal short circuit in the battery.
The object of the present invention is, in a manufacturing process of an electrode for a battery employing a three-dimensional porous metal substrate sheet, to suppress the occurrence of burrs and cuttings and the coming off of an active material while the substrate sheet is being processed. Another object of the present invention is to improve the production yield of the electrode. Still another object of the present invention is to resolve an internal short circuit in the battery, thereby preventing deterioration in the preservation performance and charge-discharge cycle life of the battery.
The present invention relates to a method for manufacturing an electrode for a battery by filling an active material into a three-dimensional porous metal substrate sheet and cutting the sheet to a certain size, comprising the steps of: (a) filling an active material into the substrate sheet, (b1) pressing a portion to be cut and the periphery thereof in the substrate sheet, and (c) cutting the substrate sheet at the portion to be cut wherein the step (b1) is conducted prior to the step (c).
This method preferably further comprises, prior to the step (c), either step of: (b2) coating the portion to be cut and the periphery thereof in the substrate sheet with a resin, or (b3) impregnating the portion to be cut and the periphery thereof in the substrate sheet with a liquid containing a resin component.
The present invention also relates to a method for manufacturing an electrode for a battery by filling an active material into a three-dimensional porous metal substrate sheet and cutting the sheet to a certain size, comprising the steps of: (a) filling an active material into the substrate sheet, (b2) coating a portion to be cut and the periphery thereof in the substrate sheet with a resin, and (c) cutting the substrate sheet at the portion to be cut, wherein the step (b2) is conducted prior to the step (c).
The present invention further relates to a method for manufacturing an electrode for a battery by filling an active material into a three-dimensional porous metal substrate sheet and cutting the sheet to a certain size, Comprising the steps of: (a) filling an active material into the substrate sheet, (b3) impregnating a portion to be cut and the periphery thereof in the substrate sheet with a liquid containing a resin component, and (c) cutting the substrate sheet at the portion to be cut, wherein the step (b3) is conducted prior to the step (c).